The invention relates to a process for the preparation of bisphenol-A prills, wherein molten bisphenol-A is charged into a prilling tower at the top of the prilling tower via a nozzle plate with a plurality of nozzles, into which prilling tower a cooling gas which is conducted in a circuit is introduced in counter-flow and wherein the bisphenol-A prill which has cooled to approximately room temperature is collected at the base of the prilling tower and is withdrawn, and moreover to a device for implementing such a process, and also to a bisphenol-A prill that is prepared in accordance with this process.
Bisphenols are chemical compounds with two phenol groups, obtained by causing phenol to react with ketones, whereby bisphenol-A (2,2-bis(4-hydroxyphenyl)propane) is formed as a result of the reaction of phenol and acetone. Bisphenol-A is processed further into epoxy resins, polycarbonates and polysulfones. With a view to alleviating pouring, transport and storage, granular material, flakes or prills are prepared from a bisphenol-A melt by cooling, whereby prills present advantages in comparison with granular material or flakes, by reason of their lower proportion of dust and better flow properties.
A process for the preparation of bisphenol-A prills is known from JP-6-107 581, wherein molten bisphenol-A is charged in the top region of a prilling tower and cooling gas, which abstracts the heat of fusion from the falling droplets of melt, is introduced in counter-flow at the base of the prilling tower. The solidified prills are withdrawn at the base of the prilling tower.
The aim of the known process was to increase the hardness of the bisphenol-A prills which are prepared, in order to reduce the proportion of dust. However, depending on the type of further processing, in addition to the very low dust content still further demands are made of prills.
The object underlying the present invention is therefore to create a process and a device for the preparation of bisphenol-A prills and to create prills which are prepared accordingly in which the purity of the product is of prime importance. In order to obtain bisphenol-A prills that are as pure as possible, on the one hand their monophenol contents and residual dust content are to be reduced, and on the other hand a particularly clear and also colour-stable product is to be created.
With regard to the process, this object is achieved by the charge of the bisphenol-A melt and the introduction of the cooling gas being effected uniformly in distributed manner over the cross-section of the prilling tower.
As far as the device is concerned, the solution is obtained by an annular line which is guided horizontally into the prilling tower and which tapers in cross-section being provided for uniform feed of cooling gas, by a baffle plate for dividing the current of cooling gas into two partial currents being provided in the radially flange-mounted cooling-gas-channel input, and by a plurality of honeycombed rectifier elements being provided, arranged uniformly over the periphery of the prilling tower. In this way it is possible, in accordance with the invention, to distribute the cooling gas in twist-free manner and directed towards the cross-section of the prilling tower.
With the process according to the invention and the device according to the invention it is possible for prills to be prepared, the dust proportion of which is below 2% by mass, which possess an inherent colour of less than 10 Haze and which exhibit a BET surface of  greater than 0.15 m2/g. The particle size dp amounts to between 0.5 and 3 mm, preferably between 0.8 and 2 mm.
The process is described more fully in the following. Essential for the process for low-phenol and low-dust preparation of bisphenol-A prills is a temperature setting of the bisphenol-A melt that is as exact as possible for nozzle atomisation within a wide working range. This temperature control is preferably made possible by means of a shell-and-tube exchanger. In the process the bisphenol-A melt is preferably obtained at 185xc2x0 C. to 250xc2x0 C. In the aforementioned shell-and-tube exchanger the bisphenol-A melt is cooled down to a temperature close to the crystallisation temperature of 156xc2x0 C. The temperature of the bisphenol-A melt which is charged into the prilling tower preferably amounts to 165xc2x0 C.
For the purpose of temperature control, according to a further teaching of the invention a 3-chamber shell-and-tube exchanger is expediently employed, whereby, with a view to cooling, the melt is passed through the tubes of the shell-and-tube exchanger. In the outer chamber around the tubes pressurised water is fed in and evaporated, in order to abstract heat from the bisphenol-A melt. The temperature of the melt is set both by the evaporation pressure of the water and via the number of the tubes which are surrounded by pressurised water. Since the radiator side of the heat exchanger is operated at below the crystallisation temperature of bisphenol-A, the heat exchanger is equipped with an additional heating jacket. During the critical phasesxe2x80x94that is to say, in the course of start-up or in low-load operation, for examplexe2x80x94freezing-up on the product side is reliably prevented by feeding a heat carrier into said heating jacket.
In a further refinement of the invention a purification of the melt is undertaken in a melt filter immediately prior to the charge of the bisphenol-A melt into the prilling tower. Metallic-cloth sieves with a mesh size of  less than 80 xcexcm are preferably employed for this filtration.
In a further refinement of the invention the nozzle plate for the charge of melt is operated under a slight excess pressure in the range from 0.05 to 1 bar, preferably between 0.15 and 0.3 bar. The differential pressure is substantially determined by the geometry of the bores, the number of bores per nozzle plate, the temperature of the melt and the feed quantity (volumetric flow rate). With a view to uniform distribution of the bisphenol-A melt, the nozzle plate is preferably spherically domed and arranged centrally on the top of the prilling tower. Moreover, in a further refinement of the invention several discharge pipes are arranged in the upper part of the prilling tower, uniformly distributed over the cross-section of the prilling tower, via which the cooling gas is withdrawn. These discharge pipes are expediently constructed with attendant heating. In this way it is possible for deposits of product to be reliably prevented.
By reason of the fact that the cooling gas is conducted in a circuit, a purification of the cooling gas has to be undertaken, which is preferably realised in the form of a bag filter.
A further teaching of the invention provides for the cooling gas which is conducted in a circuit to be continuously cooled and freed of monophenols, cooling preferably being undertaken in a washing tower with secondary cooling circuit. By this means the physical size of the cooling circuit is clearly reduced, and continuous operation is also only possible by virtue of this configuration, since, in the case of shell-and-tube exchangers which are employed for the purpose of cooling, said exchangers can always only be employed as reversible exchangers, in order to enable the phenol to be separated from the recycled nitrogen by freezing.
Cooling of the cooling gas which is conducted in a circuit is preferably undertaken with demineralised water without dissolved oxygen, in order reliably to avoid formation of foam.